The many faces of shear Alfvén waves

Abstract

One of the fundamental waves in magnetized plasmas is the shear Alfvén wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfvén waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvénic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfvén waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfvén waves. V

On the Alfvén wave cut-off in partly ionized collisional plasmas

“…The origin of differences is identified: MHD analysis in weakly ionized environment involves one small parameter = n 0 /n n0 while in the same time it misses other small parameter k 2 λ 2 i , which may be of the same order and which is naturally included only through multi-component theory. As an old subject, the Alfvén wave has already been studied experimentally in numerous works in the past, see for example [25], [26], and in particular [27] and [28] dealing with experimental partially ionized plasma; in the present work we have delivered a lot of results that should be kept in mind in eventual future experiments.…”

Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma

“…Introduction.-Shear-Alfvén (SA) fluctuations are fundamental to magnetized plasma dynamics [1][2][3]. They are routinely observed in both laboratory and space plasmas [4,5], and are the basis for modern theories of magnetohydrodynamic (MHD) turbulence [6][7][8]. They are also uniquely robust among plasma waves, with large-scale linear dynamics that are nearly unmodified across both kinetic and fluid plasma models [2].…”

Applying Alpha-Channeling to Mirror Machines

“…The ongoing fast Alfvén wave launching campaign could provide one with an opportunity to study coupling, propagation and reflection of such waves, as well as excitation of the contained fast Alfvén modes. At the same time, the LAPD device is also suitable for studying shear Alfvén waves [42]. Recently, the existence of the so-called ion-ion hybrid shear Alfvén wave resonator was demonstrated in the LAPD plasmas [43,44].…”